The invention relates generally to a method and system for treating waste materials, and in particular, to a method and system for applying very high temperatures to destroy and treat radioactive waste and other hazardous materials.
Hazardous waste disposal is a continuing problem in the United States and elsewhere. In the past, hazardous waste was most often simply buried and left in underground landfills. There was always a danger, however, that the buried waste would escape from the landfill to the environment, e.g., by leaching into ground water.
Efforts have been made to guard against environmental contamination by encapsulating solid waste inside highly durable materials. For example, solid waste has been enclosed in drums, set in concrete, and encapsulated inside environmentally stable vitreous materials. These packaging methods are problematic, however. Great care must be taken to ensure that the packaging or containers remain intact to prevent the hazardous materials from being released to the environment. This is especially difficult in the case of radioactive wastes, some of which have very long radioactive half-lives and remain dangerous for many thousands of years. Also, the packaging materials add significantly to the mass and volume of the waste itself so that a great deal of extraneous material must then be transported and buried or stored.
Additionally, previously known methods for packaging hazardous waste have not generally done much to separate the waste according to the particular severity of the hazards presented. For example, radioactive waste from a nuclear power facility might include highly radioactive waste, less radioactive but nevertheless highly contaminated construction materials, and used clothing and protective gear that may be only lightly contaminated. These various materials present different levels of danger, and may require greater or lesser care in their handling and disposal. Moreover, different materials may be amenable to treatment according to different methods. Nevertheless, these different materials are not usually separated before disposal. As a result, more waste may be disposed of and more expense incurred than would otherwise be desirable. Sorting and separating the waste, though, are difficult and potentially dangerous procedures that may subject the disposal workers to a significant exposure danger. This too adds to the cost and difficulty involved in safely and permanently storing hazardous waste.
More recently, systems and methods have been devised for destroying and treating hazardous waste with very high temperatures. For example, it has been suggested that hazardous waste be destroyed using a plasma torch, a device capable of generating temperatures on the order of ten thousand degrees centigrade.
Such very high temperature methods are capable of destroying and rendering harmless some important and highly problematic categories of dangerous waste. For example, toxic polychlorinated biphenyls (PCBs) are decomposed and rendered harmless at sufficiently high temperatures. Moreover, extremely hazardous medical waste such as xe2x80x9csharpsxe2x80x9d and other dangerous medical materials are sterilized by even relatively moderate heat.
High-temperature waste disposal methods are further advantageous in that they include an inherent separation and sorting of the waste material. At the very high temperatures used in these methods much of the waste oxidizes, pyrolyzes, and volatilizes into a hot gaseous effluent stream. The gaseous effluent stream is then treatable with conventional air pollution control apparatus. After treatment, the resulting clean gas stream can then be released to the atmosphere.
As much of the waste volatilizes away, the denser parts of the waste, consisting mainly of metals and inorganic compounds, melt to form a molten liquid melt material. This melt material may further separate into two fractions, a first fraction consisting substantially of the relatively dense molten metals, and a second xe2x80x9cslagxe2x80x9d fraction, which tends to separate and float on top of the metal fraction. If desired, the slag fraction can then be separated from the metal fraction by a variety of means and methods for further treatment and storage.
Although various methods and systems have been proposed and tried for treating hazardous waste at very high temperatures, the technology is still relatively undeveloped and much room remains for improved methods and systems. A definite need exists, therefore, for an improved method and system for treating hazardous wastes by the application of a very high temperature heat source.
It is desirable that such improved method and system provide for the controlled and continuous processing of waste so that the waste enters a high-temperature region at a steady and controllable rate. It is also desirable that the new method and system allow for whole drums or other waste containers to be processed without any substantial pre-sorting or shredding and allow for the treatment of intact drums of 115 liter, 210 liter or larger, containing a variety of wastes and varying compositions. It is desirable that the system be simple, robust, and reliable, and require relatively little maintenance and that any maintenance by simplied by modular design of system components.
It is further desired that the improved method and system facilitate convenient separation of the processed waste into its constituent parts and be operable to volatilize a large fraction of the waste to produce a gaseous effluent stream wherein the gaseous effluent stream is treatable for eventual safe release into the atmosphere. It is yet further desired that the remaining waste be separated into at least two fractions; a relatively dense molten metal fraction having a high specific gravity, and a relatively less dense xe2x80x9cslagxe2x80x9d fraction having a specific gravity substantially less than that of the metal fraction, wherein the two fractions are divisible using simple, reliable, highly controllable means into separate portions for long-term storage or disposal. These and other advantages are provided by the present invention, which is described in more detail below.
The present invention, which addresses the above need and provides the foregoing advantages, resides in a method and system for treating various types of wastes. The system includes as its primary components, a melter system and an air pollution control system. The melter system includes a number of elements and subsystemsxe2x80x94a waste feed system, a waste treatment chamber in the form of a plasma chamber, a plasma torch mounted on a plasma torch mounting assembly, a hearth, a hearth spool section, a melt collection chamber, and a secondary chamber. The air pollution control system includes an evaporative cooler, one or more pulsed-jet fabric filter baghouses, one or more high efficiency particulate air (HEPA) filter banks, a wet packed bed with full quench scrubber, a reheater, an induced draft (ID) fan, offgas recirculation systems and a stack. A reactant air supply system may also be included to assist reactions in the plasma chamber and the secondary chamber.
The system is equipped to treat whole, unopened drums of waste materials, requiring little or no pretreatment of the drums. The waste drums are loaded into a feed chamber and then fed in a slow, controlled manner by the waste feed system into the plasma chamber, where the primary processing of the entire drum and its contents occurs. Within the plasma chamber, the organic constituents of the waste are volatilized, pyrolized and/or partially combusted while the metals and other inorganic materials are incorporated into a molten pool in the hearth. The molten pool consists of both metallic and vitreous phases which are removed separately in a distinct manner. The offgas from the plasma chamber is ducted to the secondary reaction chamber where it is contacted with excess air. A natural gas burner is used to preheat the secondary chamber and provide supplemental heat and a continuous source of ignition during operation. While in the secondary chamber, the offgas is reacted with excess oxygen to further ensure destruction of substantially all remaining organic material.
After exiting the secondary chamber, the offgas is drawn from through the remainder of the system by the induced fan. The offgas is initially partially quenched in an evaporative cooler and then introduced into the baghouse for removal of the larger particulate, followed by treatment by the HEPA filter banks for removal of finer particulate. After exiting the HEPA filter banks, the offgas is substantially saturated in a water quench and passed through a packed bed scrubber for removal of the acid gases. The clean saturated offgas is then demisted and reheated above its saturation temperature prior to passing through the induced draft fan. After the induced draft fan, portions of the offgas is recirculated back the feeder chamber, the plasma chamber, and/or the scrubber, while the remaining portion is exhausted into the atmosphere through the stack.
The plasma chamber of the melter system, when operating continuously or in batches, allows for a melting (processing) mode and a collection (pouring) mode. Advantageously, the hearth is maintained static or stationary during the melting mode, so as to be devoid of substantially all motion relative to any lateral, vertical or rotational axis or plane during the melting mode. As such, disturbance within the molten bath is substantially minimized to allow the separation of the slag phase from the metal phase.
To facilitate the separation and retention of the phases, the hearth is provided with an underflow weir effectively creating a main compartment having one depth and a side compartment separated by the underflow weir at a greater depth. With separate pour spouts for the compartments, the xe2x80x9clighterxe2x80x9d slag phase and the xe2x80x9cheavierxe2x80x9d metal phase may be independently poured from the hearth with minimal cross contamination.
A tilt mechanism is provided to enable the hearth to be poured during the collection mode. With a control mechanism enabling operator-initiated and operator-controlled pouring, the tilt mechanism enables the hearth to be moved in a distinct manner combining a pivotal and translational motion which minimizes stress to the plasma chamber and disturbance to the molten bath. In one embodiment, an arrangement of pivotal links and sliding blocks enables the hearth to be substantially translated vertically, tilted in one direction, and then tilted in an opposing direction.
The hearth is configured also to provide sloped processing areas that are positioned below the feed regions where the waste materials are introduced into the plasma chamber. The sloped areas enable the waste materials to be introduced gradually into the molten pool contained within the hearth, minimizing any splashing which tends to increase the stress on the plasma chamber and its refractory lining. Whereas the hearth of the present system is stationary during the melting mode, the plasma torch is rendered mobile relative to three normal axis to ensure that the molten bath within the hearth is thoroughly treated. Multiple ground electrodes are strategically placed within the hearth such that the resulting arc may be predominantly or selectively transferred to different sites in the hearth. As such, thorough treatment of the molten bath is achieved and the pouring and collection process is facilitated. In one embodiment, a central ground and a peripheral ground are provided, the central ground being positioned somewhat in the center of the main compartment of the hearth and the peripheral ground being positioned adjacent the underflow weir. To help guide molten lead from the central ground and toward the underflow weir, a raised region circumscribing the central ground is provided in the hearth.
The melter system of the present system employs a modular design enabling the plasma chamber, the hearth and the melt collection system to be readily joined for operation of the system or separated for maintenance and the like. Overall, the system utilizes minimal movement during processing, reducing equipment failure and increasing reliability of the process.
Recognizing the aggressive nature of the treatment process, a reactant air supply system may be provided to enable flexibility and control over the atmosphere within the various chambers as appropriate for different waste materials. For example, operator-controlled air splitting between the upper and lower levels within the plasma chamber enables operating conditions (i.e. reaction stoichiometry) to be modified and varied as appropriate.
Other features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate by way of example the principles of the invention.